The present invention relates to a light-amount adjusting system that is provided in an optical apparatus such as a camera and an interchangeable lens.
A method for controlling an aperture stop (a light-amount adjusting apparatus or a light-amount adjusting unit) that is provided in an optical apparatus includes a position-servo-control method as shown in FIG. 4.
In FIG. 4, reference numeral 401 denotes an aperture control circuit, and reference numeral 402 denotes an aperture stop unit. When a target position is given to an aperture control circuit 401, a deviation that is a difference between the target position and an actual position (aperture position) of the aperture stop unit 402 is calculated. A phase and a gain of a signal indicating the deviation are adjusted by an LPF (Low Pass Filter) 403, a HPF (High Pass Filter) 404 and a gain circuit 405. Accordingly, a control signal adjusted to characteristics appropriate for the aperture stop unit 402 is generated.
An appropriate offset (OFFSET) is added to the control signal, and a limiter 406 sets a limit of a largest value and a smallest value for the added value. A PWM (Pulse Width Modulation) circuit 407 replaces the limited control signal with a PWM signal. The PWM signal is forwarded to a driver 408.
The driver 408 applies a current to a coil 409 in an aperture stop motor that drives the aperture stop unit 402 based on the PWM signal. Applying the current to the coil 409 moves a magnet 410 by an electromagnetic induction, thereby rotating a stop-blade-driving ring 411 to open/close a plurality of stop blades (light-shielding member)(not shown).
Since a change of a magnetic field associated with a movement of the magnet 410 is shown as a change of an electric signal from a hall element 412, the aperture position can be detected by the change of the electric signal. The electric signal is amplified to an appropriate value by a gain circuit 413, and used as an aperture position signal. The aperture position signal is used to generate a deviation signal showing the difference between the target position and the actual aperture position. Repeating these series of feedback operations for the aperture stop unit 402 gradually reduces the deviation, and thereby the aperture position is controlled to match the target position.
In the aperture stop unit 402, a biasing force of a spring 414 acts onto the driving ring 411 through the magnet 410 and a driving mechanism of the stop-blade-driving ring 411 in a direction of closing the stop blades.
The spring 414 provides a force to the driving ring 411 in a constant direction, thereby serving as a load component. Thus the force and torque generated by the coil 409 and the magnet 410 are readily well balanced, thereby reducing impact of disturbance from outside. In addition, the spring shields light to prevent the aperture position from easily changing when the power is off, and unnecessary light from entering.
Torque necessary for driving an aperture stop is calculated as follows. Torque T is generated in proportion to current i applied to the coil 409. When a torque coefficient is defined as K,T=Ki.
The torque T is in counterpoise with resultant forces of an inertia moment for indicating a level of difficulties about rotating a rotating member, a force caused by viscous resistance such as friction, and a force generated by the spring 414.
When the aperture position (rotating angle of an aperture stop motor) is defined as θ, the inertia force, the viscous resistance, and the biasing force of the spring 414 are in proportion to a two-time time-differential value, a one-time time-differential value, and a zero-time time-differential value, respectively.
When the inertia moment and a viscous resistance coefficient are respectively defined as J and D, a spring coefficient is defined as k, and an opening direction of the aperture stop is defined as a positive direction, a torque in the opening direction To and a torque in a closing direction Tc are expressed as follows:
            T      o        =                  K        ⁢                                  ⁢        i            =                        J          ⁢                                                    ⅆ                2                            ⁢              θ                                      ⅆ                              t                2                                                    +                  D          ⁢                                    ⅆ              θ                                      ⅆ              t                                      +                  k          ⁢                                          ⁢          θ                                T      c        =                  K        ⁢                                  ⁢        i            =                        J          ⁢                                                    ⅆ                2                            ⁢              θ                                      ⅆ                              t                2                                                    +                  D          ⁢                                    ⅆ              θ                                      ⅆ              t                                      -                  k          ⁢                                          ⁢          θ                    
As described above, the direction of the load generated by the spring 409 changes depending on the driving direction of the aperture stop. In addition, the amplitude of the load changes in proportion to the aperture position θ. An actual aperture stop includes resistive elements other than a spring which are caused by its shape and change depending on the aperture position and the driving direction thereof.
Japanese Patent Laid-Open No. 7-162741 has proposed a technique in which a negative feedback gain is changed according to a rotating direction of a motor, when a back electromotive force of an aperture stop motor is negatively feedbacked to generate a driving electric power of the motor.
As described above, a driving force necessary for controlling the aperture stop, i.e., a control characteristic changes depending on the aperture position or the driving direction. The control characteristic changes when members constituting the aperture stop changes according to a temperature change or a temporal change.
However, the conventional aperture stop is not controlled in response to those changes, thus controlling an aperture stop is unstable, and control accuracy is deteriorated.
A conventional method of generating a control signal by using the deviation between the target position and the detected position reduces the responsivity of the aperture stop in the opening direction due to the biasing force of the spring acting in the closing direction. Moreover, the force depending on the aperture position causes the aperture stop to fail to reach the target position at some aperture positions, resulting in causing a stationary deviation.